The use of fixed (stationary) facsimile devices for the transmission of image data through the PSTN (Public Switched Telephone Network) environment is known. Such fixed facsimile devices adhere to a set of protocol standards specifically designed for use in the relatively low error and low delay environment of the PSTN. In particular, one protocol specifies the manner in which the digitally represented control data is handled; another protocol specifies the manner in which the digitally represented image data is handled.
The use of analog RF communication systems, operably coupled to the PSTN, for the transmission of facsimile data between fixed facsimile devices and mobile facsimile devices is also known. Such systems use the analog RF communication system as an extension of the PSTN environment; the modem signals generated by facsimile devices and transmitted within the PSTN are also transmitted through the voiceband path of the analog RF communication system. Naturally, modem signals transmitted in such a manner are also subject to the noise and fading conditions inherent in any type of RF communication system. Thus, just as a voice message through such a system may be corrupted, the digital facsimile data represented by modem signals may also be corrupted, leading to poor recreation of the transmitted image.
Digital RF communication systems offer an alternative for the transmission of facsimile data. While such systems are also susceptible to the noise and fading conditions present in other RF communication systems, they are capable of offering protection against errors in the digital information in the form of error correction codes. The amount of error protection offered is limited in part by the available data bandwidth of such a system. Also, digital RF communication systems typically present larger delays to the transmission of message data than might be found in analog communication systems. Typically, the addition of error correction codes only add to this delay.
Attempts have been made to incorporate the protocol standards used within the PSTN directly into digital RF communication systems. It has been determined, however, that such an approach is less than satisfactory at producing acceptable image reproduction. In particular, the PSTN-based image data transfer protocol is particularly sensitive to errors; a single error can destroy an entire scan line within the image data. Error correction techniques can be used to ensure the integrity of the image data. However, such techniques often produce delays that violate timing parameters specified by the PSTN-based control data transfer protocol. For example, the GSM (Group Specialized Mobile) 3.45 standard for the transmission of facsimile data within a digital RF communication system was found to produce unacceptable image quality. The subsequent GSM 3.46 standard is extremely complex and timing sensitive, as well as non-transparent to the facsimile devices used. Therefore, a need exists for a solution that allows transparent transmission of facsimile data, with respect to the facsimile device, through a digital RF communication system, and having acceptable image quality.